Properly deprecate config.load_paths and config.gem

railties/lib/rails/engine/configuration.rb

@@ -48,6 +48,33 @@ module Rails
       def autoload_paths
         @autoload_paths ||= paths.autoload_paths
       end
+
+      def load_paths
+        ActiveSupport::Deprecation.warn "config.load_paths is deprecated. Please use config.autoload_paths instead."
+        autoload_paths
+      end
+
+      def load_paths=(paths)
+        ActiveSupport::Deprecation.warn "config.load_paths= is deprecated. Please use config.autoload_paths instead."
+        self.autoload_paths = paths
+      end
+
+      # Rails 3, by default, uses bundler, which shims the Kernel#gem method so that it should
+      # behave correctly for this deprecation.
+      def gem(name, options = {})
+        super name, options[:version] || ">= 0"
+        require options[:lib] || name
+      rescue Gem::LoadError
+        msg = "config.gem is deprecated, and you tried to activate the '#{name}' gem using it.\n"
+
+        if File.exist?("#{Rails.root}/Gemfile")
+          msg << "Please add '#{name}' to your Gemfile."
+        else
+          msg << "Please update your application to use bundler."
+        end
+        ActiveSupport::Deprecation.warn(msg, caller)
+        exit
+      end
     end
   end
 end